In the recovery of hydrocarbons from hydrocarbon reservoirs, wells are drilled from the seabed or the earth's surface down to and into the reservoir which is under pressure. The well is lined with casing to prevent it from caving in, and placed inside the casing is a tubing that extends from the wellhead on the seabed or the earth's surface into the reservoir. The casing is perforated in the reservoir to enable hydrocarbons to flow into the casing and then into the tubing and up to the wellhead for further treatment.
A hydrocarbon reservoir may contain oil, gas and water. The production conditions, that means primarily the amount of oil, gas and water and the pressure in the reservoir, usually vary through the reservoir and alter during the course of the production time. To be able to control production from the well, that is to say control the inflow of oil, gas and water in the well, it is desirable to be able to shut off and admit the inflow to the well at different points along the tubing.
Water or gas injection is used in some places to maintain the pressure in the reservoir, that is to say that pressurised water or gas is forced into the reservoir from the well, and in that case it may be desirable to regulate the outflow from the well along the tubing.
Sleeve valves, which can be placed at suitable intervals along the well in the reservoir, can be used to control the flow to or from a well. The sleeve valves comprise an outer sleeve and an inner sleeve that are both provided with openings. The outer sleeve is fixed and forms a part of the tubing, whilst the inner sleeve is movable to align and misalign the openings in the two sleeves in order to admit or shut off flow through the sleeve valve.
The reservoir may extend across a very large area, for example, 2000 metres, and is usually divided into different production zones which may have very different pressure. The tubing extends through the different production zones, and may have one or more sleeve valves in each production zone, so as to enable hydrocarbons to be produced from one or more production zones, whilst other production zones may be closed. The production zones can be separated from one another by isolation packers that are placed between the casing and the tubing to prevent fluids from leaking between the production zones along the outside of the tubing.
If there is production from a production zone at a pressure of, for example, 200 bar, the tubing will have an internal pressure of approximately 200 bar. If the tubing passes through a production zone where the pressure is, for example, 50 bar, the sleeve valves must be closed in this production zone to prevent outflow from the well. The sleeve valves are thus subjected to a differential pressure of 150 bar between the outside of the outer sleeve and the inside of the inner sleeve.
The reverse may also happen, that is to say that there is production from a production zone having a pressure of 50 bar, and the tubing passes through a production zone where the pressure is 200 bar, where the sleeve valves in order to prevent inflow are closed. In that case, the sleeve valves are also subjected to a differential pressure of 150 bar, in a direction opposite to that of the aforementioned differential pressure.
A high differential pressure in itself can cause leakage. In addition, differential pressure can lead to deformation of the sleeve valves, which can cause the sleeve valves to leak when closed.
The problem associated with leakage of the sleeve valves because of the substantial differential pressures can be solved by using flexible gaskets of a non-metallic material. However, the temperature in the reservoir can be 100° C. or more, and the reservoir may contain constituents that corrode the gasket materials. In sleeve valves where the sealing is based on gaskets, it has been found that leakage often occurs after some time when the valve is in a closed position, and gaskets are thus not a satisfactory solution to the problem.
To avoid gaskets, valves having metal-to-metal seals have been developed. However, metal-to-metal seals have little capacity to absorb the large deformations that can be caused by substantial differential pressures, and therefore also do not provide an adequate solution to the problem of sleeve valves that leak in a closed position when subjected to substantial differential pressures.
GB 2 201 979, U.S. Pat. No. 4,782,896 and U.S. Pat. No. 4,921,044 describe sleeve valves for controlling fluid flow between a hydrocarbon reservoir and a well in the hydrocarbon reservoir, comprising a fixed outer sleeve and an inner sleeve that is movable between positions in which openings in the outer sleeve and the inner sleeve are aligned or misaligned.